Page 76 - ICSE Chemistry 6

• Water on the surface of the earth: About 70 per cent of the earth’s surface is covered with water.
About 97 per cent of the earth’s water is present in oceans and seas. The remaining 3 per cent
is fresh water. Most of the fresh water is found in the form of ice or snow in the polar regions,
mountains and glaciers. About one-third of the fresh water is present in rivers, lakes, springs,
ponds, etc. This water is used for agriculture and drinking purposes.
• Water in the atmosphere: Water is present as water vapour, clouds, mist, etc. in the atmosphere.
• Water in living beings: About 70 per cent of the human body consists of water. Water is an
important part of our diet because it helps the body to perform specifi c metabolic tasks. The fruits,
vegetables, foodgrains, etc. that we take contain water.

Activity 4.1

Aim: To fi nd the water content in diff erent food items
Use the Internet and fi nd the water content in the following food items.

• Cucumber • Milk • French beans • Tomato
• Orange • Potato • Wheat fl our • Bread
• Rice • Mango • Watermelon • Apple

SOURCES OF WATER

The major sources of water can be divided into the following categories.
• Rainwater • Surface water • Groundwater

Rainwater

Rain forms an important part of the water cycle. Rainwater is the purest
form of water. It replenishes other sources of water. However, while falling
down, it gets polluted with dust par cles and some dissolved gases such as
oxides of carbon, nitrogen and sulphur present in our atmosphere due to
human ac vi es.

Surface Water Fig. 4.2: Rain

Surface water consists of water present in the following places.

• Oceans and seas: They cover about 70 per cent
Thirst for Knowledge
surface of the earth. Water present in oceans and
seas contains a large amount of dissolved salts. Due • The salinity of sea water is 35 g/kg.
to the presence of dissolved salts, this water is salty • The salinity or dissolved salt content
and is called saline water. It is unfi t for drinking, of a solu on can be measured with a
device called salinometer.
washing, irriga on and industrial purposes.
• Rivers, lakes, streams and ponds: Water present
in these water bodies comes from the mel ng of snow on mountains and from rain. Along its
course, water travels through hills and plains, and becomes impure. It contains dissolved salts and
suspended impuri es.

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